Electrostatic switched radiator for space based thermal control

ABSTRACT

A thermal control device for controlling the temperature of a craft/spacecraft by means of an electrostatic switch to change the mode of heat transfer of the craft/spacecraft skin from conduction to radiation. The change is by means of a large change in apparent emissivity. The device can operate with moderate levels of DC voltages. Application of voltage results in high emissivity while removal of voltage results in low emissivity.

FIELD OF THE INVENTION

[0001] The present invention relates to an improved device forcontrolling the effective emissivity of a surface by means ofelectrostatic attraction, which controls the thermal conductivity.

BACKGROUND OF THE INVENTION

[0002] Control of solar absorption and/or thermal emissivity isimportant for temperature control involving systems where radiation isthe major heat control mechanism. Control of black body radiation andsolar absorption, using a spectrally selective coating, will helpcontrol the temperature. But, when the heat load varies, active controlof the thermal radiation is needed. Coolants have been used to conductheat to an external radiator and can be controlled to block, or to beopen, to piping. Louvers are another alternative that can be used toopen or close. With a louver in one position, the exposed surface willhave a high emissivity; alternately when the louvers is in the otherposition, the exposed surface will have a lower emissivity and willradiate less heat. When radiators are fixed, as in present art, optionsincluding heat pipes, heat pump systems, capillary pump looped heatpipes and louvers can be effective but are expensive, heavy and bulky.

[0003] Electrostatic forces have been used previously in variousapplications.

[0004] U.S. Pat. No. 4,665,463 (1986) to Ward et al. describes anelectrostatic chuck for holding a semiconductor wafer, comprising adielectric layer on a supporting electrode. A potential is appliedbetween the wafer and the electrode and the dielectric is loaded withthermally conductive material to improve dissipation of heat generatedin the wafer during a processing treatment such as exposure to anelectron beam.

[0005] U.S. Pat. No. 4,771,730 (1987) to Tezuka et al. describes avacuum processing apparatus with a vacuum vessel within which a work tobe processed is drawn and held fixed on a specimen table by an electrodefunctioning doubly as an electrostatic chuck, to which is connected agas feeding pipe for feeding a gas affording good heat transmissionbetween the mutually contacting surfaces of the work and the electrodeto control the temperature of the work.

[0006] What is needed is a smaller, less expensive, flexible, lighterweight, higher performance, and more reliable solution. The presentinvention solves these problems with use of an electrostatic switchedradiator.

SUMMARY OF THE INVENTION

[0007] The main aspect of the present invention is to provide anelectrically switched radiator for space based thermal control by meansof an electrostatic hold-down or release of a thin composite film tocontrol inner compartment craft/spacecraft temperature.

[0008] Another aspect of the present invention is to provide thermalcontrol by producing a large change in effective emissivity whenswitching the device from the “off” (non-radiating) to the “on”(radiating) stage.

[0009] Another aspect of the present invention is to provide a highemissivity composite film to control craft/spacecraft skin temperature.

[0010] Another aspect of the present invention is to provide a means forswitching the effective emissivity from a low to a high value and visaversa via contact/non-contact with a surface to be cooled.

[0011] Another aspect of the present invention is to provide for a thincomposite film which is flexible for good contact with the outer skin ofthe craft.

[0012] Another aspect of the present invention is to provide a low cost,low weight, high performance, high reliability and small sizeelectrostatically controlled radiator for thermal control ofcraft/spacecraft temperatures.

[0013] Other aspects of this invention will appear from the followingdescription and appended claims, reference being made to theaccompanying drawings forming a part of this specification wherein likereference characters designate corresponding parts in the several views.

[0014] The present invention utilizes a high emissivity composite filmto control craft/spacecraft skin temperature. The electrostatichold-down switches the mode of heat transfer from the craft/spacecraftskin from a conduction mode to a radiation mode and back. The device isreferred to as the “Electrostatic Switched Radiator” or ESR. The ESR isvery lightweight and has demonstrated with experimentation thecapability to switch the emissivity from below 0.1 to above 0.95.Emissivity is simply the ratio of the actual emitted radiance to that ofan ideal blackbody. Emissivity ranges from 0 to 1 where 1 would be ablackbody. Emissivity can also vary with wavelength for any particularsubstance. For example, the emissivity for a water droplet decreases asthe wavelength decreases.

[0015] The ESR construction is simple and lightweight. It consists of athin polymer film, with a weight of less than a few hundred grams/m².The film can be anchored to the craft/spacecraft at the edges. The coverfilm consists of a high dielectric constant insulator with a gooddielectric strength and is coated on its outer surface with anelectrically conductive thin layer. The outer surface of the ESR isconstructed to have a very high emissivity, ideally with low visibleabsorbance. This combination can be achieved with an appropriate paintor, for better performance, a multi-layer thin film designed for verylow visible absorbance and high emissivity. The top surface or “skin” ofthe craft/spacecraft, to which the ESR will be “in contact” or “not incontact” should have a very low emissivity, i.e. sputtered gold.

[0016] Basic heat control is simple and highly effective. When the ESRis turned on, the emitting surface is in good thermal contact with aninner surface skin (such as the outer skin of a spacecraft). Thisresults in good heat conduction between the craft surface (skin) and theESR such that the emitting surface of the film is at the craft skintemperature. The emitting ESR surface radiates at the “skin” temperature(high emissivity state). When the ESR is turned off, the film moves awayfrom the skin (is not in contact) and the heat flow is only radiationfrom the inner surface skin (low emissivity). Thus, once it reachesequilibrium, the film can only radiate the heat it absorbs which islimited to radiation from the inner surface skin. The inner surface skinis fabricated with a low emissivity and thus in the released state, theouter skin emissivity doesn't change, however it's temperature drops andthe result is a drop in the radiated energy. This approach avoids theneed for an infrared (IR) transparent conductor, which is alwaysdifficult since transparent conductors (wide band gap semiconductorswith high electron concentration) have significant absorption in the IR.

[0017] The ESR requires minimal material requirements and the system iscompatible with conventional paints and coatings for full utilization inlow solar absorbance, high emissivity coatings. Typical films couldconsist of an outside coating of copper with only 1000 angstrom to 25-50micron thickness. A sputtered metal on a polymer will improve hold-downvia a more pliable structure and allow operation at a lower appliedvoltage. The film itself is an insulator such as polyimide (Kapton) andrequires a high dielectric constant, high thermal conductivity, and highdielectric strength properties. Other films such as Kynar are alsoalternatives. The ESR operates as a high quality capacitor with adielectric (film) between two layers of metal (film metallic coating andcraft metallic coating). The surface area (radiating area) of the ESR iscalculated to dissipate heat needed to control the internal temperatureof the vehicle or craft. Surface area is selected as a function of heatgenerated in order to determine the amount of heat to be radiated. TheESR can be subdivided into sectional areas depending on designrequirements. The skin or area of contact, such as the outer skin of acraft, is required to be metallic or metallic coated (typicallyaluminum). Typical internal craft temperatures are often controlledaround 300 degrees Kelvin (Room Temperature). The heat generated insidewill raise the internal temperature and the ESR will dissipate the heatto control the internal temperature to room temperature.

[0018] Switching a DC voltage controls the “on” versus the “off” stateof the ESR. The ESR will operate effectively with moderate levels of DCvoltage (typically 100-500 VDC). When voltage is applied between theouter conductive surface of the film and the outer conductive of thecraft, the film is attracted to the craft surface and a high emissivitylevel results transferring heat. Experimentation has shown that theapplied voltage can be lowered significantly before the film releases.Removal of the voltage results in physical separation and thus a lowemissivity state. Experimentation has shown that physical separation byfloating was acceptable. Other means of insuring separation can beachieved with a release mechanism. The coated film can be attached atall corners or simply at one edge. A piezoelectric strip attached to thecover film in which applying a voltage would cause them to expand,causing the entire structure to bend (bimorph). Other approaches wouldbe nonconductive hinges, slight tension on the composite film at theedges so as to control “spring-back”, a spring loaded or magneticallyactuated plunger that simply moves the cover film out a small amount,etc.

[0019] Early experimentation by the inventor has shown individual ESRdevices can be fabricated with measured emissivity changes of 0.74 ormore. Additionally, it has been shown that ESR devices can be fabricatedwith achieved high value emissivity levels greater than 0.9 and lowvalue emissivity levels of lower than 0.1. Thus devices with thesecharacteristics would generate a change of greater than 0.8 in switchedemissivity levels.

[0020] Actual test measurements of a working device contained within avacuum bell jar were performed using a copper block with an areaconsisting of a flat black painted strip as the high emissivityreference, an area of bare copper as a low emissivity reference, and anarea with the ESR. Measurements were taken using an Inframetics 625imager, which is a camera sensitive from 8 to 14 microns. Test resultsshowed that, within the limits of the test setup, the “on” state of theESR was approximately the same as the black painted substrate and the“off” state of the ESR was approximately the same energy as the barecopper. The black painted area was estimated to have an emissivity ofapproximately 0.95 and the bare copper an emissivity of less than 0.01.Thus, within the limits of the experimental measurements, the highemissivity “on” state (with electrostatic hold-down) was shown to havethe same emissivity as the black painted strip and the low emissivity“off” state (with no electrostatic hold-down) was shown to have the sameemissivity as the bare copper. This test showed that electrostatichold-down would insure good thermal contact and that a vacuum systemcould produce a sufficiently low pressure to eliminate or minimizethermal conduction from the air.

[0021] Since this measurement is sensitive to the wavelength of thedetector, additional measurements were made in which measurements of theheat loss were used to determine the emissivity. With this measurement,the sample is placed on a thermal control plate and the heat loss ismeasured by measuring the power required to maintain temperature and isbasically a calorimetric approach. The heat loss with ESR switchinggives a very accurate measure of a change of emissivity. Absolute valuesof emissivity require a calibrated sample, which used a sputtered goldfilm as the “zero” emissivity point and a black paint for the highemissivity value (ε.˜0.9).

[0022]FIG. 3 (below) shows measured results for a sample consisting of acover film with a thin aluminum film. For this measurement, the voltagewas applied while the sample was warm and showed a change of theeffective emissivity of 0.74. This test showed that electrostatichold-down would insure good thermal contact.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1A, 1B is a depiction of the ESR in the “OFF” and the “ON”states, respectively.

[0024]FIG. 2A is a cross-sectional view of the ESR with a voltage sourceand switch and the ESR in the “OFF” position.

[0025]FIG. 2B is a cross-sectional view of the ESR with a voltage sourceand switch and the ESR in the “ON” position.

[0026]FIG. 3 is a graph of power input measurements of a thin metallizedESR.

[0027]FIG. 4 is a depiction of a craft with two ESRs attached, one inthe “ON” and one in the “OFF” position.

[0028] Before explaining the disclosed embodiment of the presentinvention in detail, it is to be understood that the invention is notlimited in its application to the details of the particular arrangementshown, since the invention is capable of other embodiments. Also, theterminology used herein is for the purpose of description and not oflimitation.

DETAILED DESCRIPTION OF DRAWINGS

[0029]FIG. 1A is a depiction of the ESR 1000 in the “OFF” state. Theouter skin 100 of a craft radiates heat via radiation Rs. Thermal gap Gacts as an insulator. Energy is radiated Rs by the outer skin 100, whichhas a low surface emissivity. Heat is absorbed by the composite film 102consisting of the dielectric 101 and the thin metallic surface coat 104.In this “OFF” position, the heat loss from the outer skin 100 is theenergy lost from radiation emitted Rs minus the energy absorbed from thereflected radiated energy Ra of the composite film 102 (dielectric 101and thin metallic surface coat 104) and minus the energy absorbed fromthe radiated energy Rb by the surface of the composite film 102(dielectric 101 and metallic surface coat 104) at an unknowntemperature. The composite film 102 (dielectric 101 and outer thinmetallic surface coat 104) thus emits reflected radiated energy Ra backto the skin 100 and radiated energy Rb from the surface temperature.Energy is also transferred into space by radiated energy Rc from thehigh emissivity surface of metallic surface coating 104. A steady statetemperature will occur when the temperature of the dielectric 101 andmetallic surface coat 104 reaches a temperature such that it's energyabsorbed is equal to its energy radiated.

[0030]FIG. 1B is a depiction of the ESR 1000 in the “ON” state. In thisstate, the outer skin 100 of a craft is in close contact 103 (thermalcontact) with the thin film dielectric 101 and its metallic coating 102.In this “ON” state heat is conducted away from the outer skin 100 of thecraft at a very high level of efficiency and transferred into space byradiation Rc off the metallic surface 104 which is designed at a highemissivity.

[0031]FIG. 2A is a cross-sectional view of the ESR 1000 with a voltagesource 204 and switch 205 open and thus the ESR 1000 in the “OFF”position. The metallic surface coat 104 and thin film dielectric 101 areseparated from the outer skin 100 of the craft by a thermal gap G.Non-conductive hinges 201 could be used to facilitate the separation, ascould a piezoelectric strip. A DC circuit consists of a DC voltagesource 204, a switch 205 in the open position, an outer skin contactpoint 207, a connector 202 and insulator 206 and a wire contact 203 tothe metallic surface coat 104.

[0032]FIG. 2B is a cross-sectional view of the ESR 1000 with a voltagesource 204 and switch 205 in the “ON” position. In this position thethermal gap G is basically zero. Thus, the metallic surface coat 104 andthin film dielectric 101 are in direct contact with the outer skin 100of the craft. Various methods are possible to insure that this gap G issufficient to limit the heat transfer. This can include non-conductivehinges 201 as shown, which are designed to collapse. Normal elasticityof the cover film when stretched and mounted at the ends may besufficient to insure such a gap. A piezoelectric strip attached to thecover film as previously described could also be used. In this “ON”position the DC voltage source 204 pulls the thin film dielectric 101and its metallic surface coat 104 into thermal contact with the outerskin 100. The ESR 1000 transfers heat from the outer skin 100 andradiates the heat into space by the high emissivity surface of themetallic surface coat 104. As also shown in FIG. 2A, is a DC circuitconsisting of a DC voltage source 204, a switch 205 in the closedposition, an outer skin contact point 207, a connector 202 and insulator206 and a wire contact 203 to the metallic surface coat 104.

[0033]FIG. 3 is a graph of power input measurements of a thin metallizedESR as was discussed above; the sample tested having a change ofemissivity of 0.74. When the ESR was switched from a low “off” state 300to a high “on” state 301 it can be seen that power requirementsdecreased

[0034]FIG. 4 is a depiction of a craft 400 with two ESRs attached, onein the “ON” position 405 and one in the “OFF” position 406. Energyreleased within a craft can come from electronics 403 and/or humanoccupants 404 which would increase the internal craft temperature if nocontrol were present. Electronic switching sensors 401, 402 would allowfor electrostatic switching of the ESRs 405, 406 by allowing a voltageto be “on” or “off”. Contact points for the electronic sensor 401 are atthe outer skin 408 and at the metallic surface 407 of the ESR 406, whichis shown in the “OFF” or non-contact mode. Thus, little or no energy isradiated into space S as previously discussed. Contact points for theelectronic sensor 402 are at the outer skin 410 and at the metallicsurface 409 of the ESR 405, which is shown in the “ON” or contact mode.Thus, energy Rc is radiated into space S as previously discussed.

[0035] Although the present invention has been described with referenceto preferred embodiments, numerous modifications and variations can bemade and still the result will come within the scope of the invention.No limitation with respect to the specific embodiments disclosed hereinis intended or should be inferred.

I claim:
 1. A radiator comprising: an object having a low emissivityouter layer; and a movable covering having a contact mode and anon-contact mode with the low emissivity outer layer, thereby enabling ahigher amount of heat to radiate from the object in the contact moderelative to the non-contact mode.
 2. The radiator of claim 1, whereinthe low emissivity outer layer further comprises a skin and the objectfurther comprises a craft usable in space.
 3. The radiator of claim 2,wherein the movable covering further comprises an outer high emissivitylayer and an inner layer comprising a dielectric.
 4. The radiator ofclaim 3 further comprises a switchable electric power source having aconnection to the skin and a connection to the outer high emissivitylayer, wherein in a powered mode an electrostatic attraction causes thecontact mode.
 5. The radiator of claim 4 further comprising a separatorfunctioning to urge the movable covering to the non-contact mode.
 6. Theradiator of claim 4, wherein the outer high emissivity layer furthercomprises a thin metallic coating and the dielectric further comprises afilm having a high dielectric constant, a high thermal conductivity anda high dielectric strength.
 7. The radiator of claim 6, wherein theswitchable electric power source is a DC source.
 8. A radiatorcomprising: a craft having a low emissivity outer layer; a movablecovering having a contact mode and a non-contact mode with the lowemissivity outer layer; said movable covering further comprising acomposite film with an inner dielectric base and an outer highemissivity metallic coating over the inner dielectric base; a switchedpower source having a first pole connected to the low emissivity outerlayer and a second pole connected to the high emissivity metalliccoating; and wherein a non-powered state of the outer high emissivitymetallic coating causes the non-contact mode and a low heat transferrate away from the craft, and a powered state of the outer highemissivity metallic coating causes the contact mode and a high heattransfer rate away from the craft.
 9. The radiator of claim 8, whereinthe movable covering is flexible.
 10. The radiator of claim 9, whereinthe craft is located in space.
 11. The radiator of claim 8, wherein theswitched power source is DC.
 12. A variable heat transfer surface, saidsurface comprising: a low emissivity outer layer covering at least aportion of a heat-emitting craft; a movable covering having a contactmode and a non-contact mode with the low emissivity outer layer; saidmovable covering further comprising a composite film with an innerdielectric base and an outer high emissivity metallic coating over theinner dielectric base; a power source connected across the lowemissivity outer layer and the high emissivity metallic coating; aswitch to supply power “ON” and “OFF” across the low emissivity outerlayer and the high emissivity metallic coating; and wherein the switchin the “OFF” position causes the non-contact mode and a resulting lowheat transfer rate away from the surface, and the switch in the “ON”position causes the contact mode and a resulting high heat transfer rateaway from the surface.
 13. The variable heat transfer surface of claim12, wherein the low emissivity outer layer further comprises at least aportion of a craft, said craft being usable in space.
 14. The variableheat transfer surface of claim 13, wherein the movable covering isflexible.
 15. The variable heat transfer surface of claim 12, whereinthe movable covering is flexible.
 16. The variable heat transfer surfaceof claim 12, wherein the power source is DC.
 17. A radiator comprising:a low emissivity outer layer means functioning to cover at least aportion of a craft; a temperature control means functioning to controlthermal emissivity from the craft; and said temperature control meansfurther comprising a movable covering having a contact mode and anon-contact mode with the low emissivity outer layer means, therebyenabling a higher amount of heat to radiate from the craft in thecontact mode relative to the non-contact mode.
 18. The radiator of claim17, wherein the movable covering further comprises a flexible compositefilm means further comprising an inner dielectric base means and anouter high emissivity metallic coating means functioning to cover a lowemissivity outer layer means.
 19. The radiator of claim 18, wherein themovable covering further comprises a switched power source having afirst pole connected to the low emissivity outer layer means and asecond pole connected to the high emissivity metallic coating of thetemperature control means.
 20. The radiator of claim 19 furthercomprising a DC power source means functioning to draw together the lowemissivity outer layer means and the temperature control means via anelectrostatic force.
 21. A radiator comprising: a low emissivity outerlayer means functioning to cover at least a portion of a craft; atemperature control means functioning to control thermal emissivity fromthe craft; said temperature control means further comprising a movablecovering having a contact mode and a non-contact mode with the lowemissivity outer layer means, thereby enabling a higher amount of heatto radiate from the craft in the contact mode relative to thenon-contact mode; wherein the movable covering further comprises aflexible composite film means further comprising a inner dielectric basemeans and an outer high emissivity metallic coating means functioning tocover a low emissivity outer layer means; wherein the movable coveringfurther comprises a switched power source having a first pole connectedto the low emissivity outer layer means and a second pole connected tothe high emissivity metallic coating of the temperature control means;wherein the movable covering further comprises a switched power sourcehaving a first pole connected to the low emissivity outer layer meansand a second pole connected to the high emissivity metallic coating ofthe temperature control means; and a DC power source means functioningto draw together the low emissivity outer layer means and thetemperature control means via an electrostatic force.